1. Field of the Invention
This invention relates to release coating compositions, and, more particularly, to UV-curable liquid compositions of predetermined viscosity containing an acrylate-functional silicone resin and a miscible vinylether.
2. Description of the Prior Art
Release coatings are substances which control or eliminate the adhesion between two surfaces. Often, curing of the liquid release coating is accomplished by solvent removal and thermal crosslinking. Radiation curable release coatings, on the other hand, contain no solvent and have the unique ability to be cured at, or slightly above, room temperature. Polymerization is initiated by ultraviolet light (UV) or electron beam (EB) exposure. Most commercial radiation curable release coatings are based on polydimethylsiloxane resins which have been functionalized with reactive groups.
Many successful radiation curable release coating are based on acrylate functional silicones which are cured by a free radical mechanism. The viscosity and cure speed of these systems are best modified by incorporating reactive diluents; however, the choice of reactive diluents is severely limited by the poor compatibility of many commonly used monomers. Furthermore, free radical polymerizations are inhibited by the presence of oxygen and the high oxygen permeability of silicones amplifies this effect. Thus, commercially acceptable cure speeds are best obtained with expensive nitrogen inerting. Cationically curable epoxy and vinyloxy organo polysiloxanes also are known. These coatings cure rapidly with UW exposure, and, unlike free radical polymerizations, cationic polymerization is not inhibited by the presence of oxygen, thus eliminating the need for nitrogen inerting. Nevertheless, there are still several performance needs. First, compatible reactive diluents are still needed to adjust the viscosity and cure speed. Second, while UW-curable silicone release coatings can readily provide a low energy surface giving low or "easy" release coatings; controlling the level of release is difficult. Control release additives have been only partially successful in developing moderate or "tight" release coatings. Finally, silicones are frequently less economically attractive than other organic resins.
Vinylethers are effective reactive diluents for epoxy functional silicones in cationic release coating systems. Vinylethers are also known to be effective reactive diluents for urethane and epoxy acrylate oligomers in free radical and hybrid wood coating formulations.